Diversity receiver for multiplex frequency shift tones



Feb. 27 E951 H o PETERsoN 2,543,256

DIVERSITY RCEIVER FOR MULTIPLEX FREQUENCY SHIFT TONES Filed May 20, 1948 3 Sheets-Sheet l Feb. 279 1951 H. o. PETERSON 2,543,256

DIvEEsITY RECEIVER EoR MULTIPLEX 4FREQUEMY SHIFT ToNEs Filed May 20, 1948 3 Sheets-Sheet 2 l INVENTOR O. T HARO D /E/:ESON

Feb., 279 M51 H. o. PETERSON DIVERSITY RECEIVER RoR MULTIRLRX FREQUENCY SHIFT TUNES 3 Sheets-Sheet 5 Filed May 20, 1948 xNvr-:N-roR HAROLD 0. PETERSON BYAuJLz/k' A ORNEY Patented Feb. 27, 1951 DIVERSITY RECEIVER FOR MULTIPLEX FREQUENCY SHIFT TUNES Harold 0. Peterson, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application May 20, 1948, Serial No. 28,254

(Cl. Z50-8) Claims. 1

This application relates to diversity receiver systems in general, and in particular to receivers of telegraph or Teletype signals or facsimile signals or the like. Telegraphy, Teletype or facsimile signals may be of the on-olf type but are preferably of the frequency shifted type and in describing my invention, reference will be made to systems wherein the signals are represented by frequency shifted currents of tone frequency, the latter being used to amplitude or phase modulate a carrier of higher frequency for transmission purposes.

It is well known that fading effects at different points and in antennas of different character at the same point are different so that the same signal at one of said antennas may be better than at the other antenna, and I make use of such diversification in order that better signals may be obtained in the receiver output. In this respect, my system has much in common with the two-set space diversity system with fast acting channel selecting means shown in Schock et al. application Serial #632,978, led J'Jecernber 5, 1945, now Patent No. 2,515,668, granted July 18, 1950.

In some cases, radio telegraphy signals and the like are transmitted over radio telephone channels. The radio telephone channels are of about 6,000 cycles width and the on-off telegraph channels each require a band of frequencies of about 2,500 cycles wide and under'the best conditions only two on-oif telegraphy channels can be set up over one radio telephone channel. It is of obvious importance to make the best use of the frequency spectrum and to this end, it has been 'r found desirable to transmit radio printer channels by frequency shift modulation of tones transmitted over the radio telephone circuit in this multiplex system. The radio telephone circuit may operate on either single side band or double side band modulation methods and the transmitted carrier may be either phase or amplitude modulated by the frequency shiftedtones. In the present application, it is assumed that the frequency shifted tones are used to amplitude or phase modulate the carrier and the carrier and one or both side bands are transmitted. At the receiver, the frequency shifted modulated tones are recovered and demodulated toderive the signal.

An object of the present invention is to provide an improved receiver adapted to receive and demodulate signals of the character described above. This system includes two receivers in a novel diversity system.`

Distortion in signals results in large part from improper relation between the carrier and side band magnitudes in the demodulator and a further object of my invention is to provide a demodulation system for this type of transmission wherein the demodulator in each receiver is of the exalted carrier type. This object is attained by use of a novel gating system for selecting the best carrier picked up by the two receivers, which carrier is then used in the demodulators of both receivers. Thus, the best carrier is always available in the demodulators used for recovering the several tones.

The tones derived by the demodulators are then isolated and the best tone on each channel is selected by a signal strength sensing and detecting arrangement with a locking circuit controlled gating system. Thus, by the use of exalted carrier reception and diversication of each tone channel in the receiver outputs, the best possible quality of received signals results at all times.

In describing my invention in detail, reference will be made to the attached drawings, wherein Fig. 1 illustrates by circuit element and circuit element connections a diversity receiver system for multiplex tone signals on a carrier arranged in accordance with my invention. In this figure, for the sake of simplicity, rectangles are used to show the various components of the receiver. Where the circuit connections included in the rectangles need illustration, I have shown the same in separate figures.

Fig, 2 illustrates the essential features of a gating system orated by a locking circuit in turn controlled by a signal strength comparing and detecting circuit, the arrangement of the comparing and detecting circuit and locking circuit being as disclosed in said U. S. patent referred to above. The arrangement of Fig. 2 might replace the carrier selector of Figs. 1 and 4 or the audio channel selector of Figs. 1 and 4.

Fig. 3 illustrates a converting detector appropriate for use in the rectangle I9 and i9 of Fig. 1.

Fig. 4 is a modification of the arrangement of Fig. 1. In Fig. 4, in order to operate the apparatus for selecting the best tone at higher frequency, l have included a frequency multiplier in each channel between the channel lter and diversity selector and a frequency divider between the limiter and lter in the common output.

In Fig. l, A and A represent spaced antenna systems for intercepting radiant energy waves. The antenna systems may be directive and are in space diversity and supply current representing supplied to first intermediate frequency amplifierI These intermediate frequency. amplifiers comprise, in addition to circuits with` units 1F and IF.

the necessary selectivity and gain, a converter wherein the intermediate frequency energyV is'v mixed with oscillatory energy from a second'local oscillator LO' of a frequency such as to supply at the outputs of IF and IF currentsor. voltages. of low intermediate frequency which carry the.

multiplex signals. In the example given, the ose cillator LO" is assumed to operate at 400 kc. per second in which case the second intermediate frequency outputs of IF and IF are at 50 kc. The output of each of the first intermediate frequency stages IF and IF is supplied to two branches one of which, l@ and I', is designated the wide branch and the other of which, I2 and I2', is designated the narrow branch. In the wide branch, I include lters or band pass circuits-of such a character as to pass without material attenuation the carrier and one or both side bands depending on whether double or single side band operation isY to be carried out. This wide band branch supplies carrier and side band energy by leads v I5 and I5 to demodulators Iii-and I9. These demodulators or detectors are of the frequency converter type and their outputs include the frequency shifted audio tones used as modulation on the carrier in the transmitter.

The narrow branches i2 and I2' include sharp or narrow band filters or tuned circuits having characteristics such that substantially the carrier only is extracted or passed-and the side bands are materially reducedl or removed so that substantially only carrier energy is supplied by lines I 4 and I4 to the apparatus in the unit designated I8. This selector unit I8 comprises a carrier strength comparing and/or sensing detector arrangement, a locking circuit,.a control tube connecting the detector output to the locking circuit, a pair of gating tubes and connections between the locking circuit output and the gating tubes such that the best carrier-coming in on lines I4 and I4' is selected and supplied as output to the lines 20 for use in the demodulators of each receiver.

The carrier energy which is now an exalted carrier is then passed through the limiter 22 and a narrow lter 24 to remove therefrom any modulation frequencies introduced by the switching action in the limiter 22 and selector i 8. This filtered carrier is supplied through leads 25 and 28 and 28' to the converters I 9 and I9 wherein it is used to demodulate the 50 kc. signalcarrier currents supplied to demodulators I9 and I9' from the wide branch filters I0 and I'. The several audio channels are then separated by audio channel filters 35, 3V', 38, 33 and so forth. Each tone appears at the output of both receivers and the best version of each of the tones is selected in the selectors 46, de' and so forth. Lines 31 and 31' feed the respective outputs of the lters 36 and 36' for channel #I to selector 40, for example. The selectors 4i), 4G and so forth may each" in general be similar to that in unit I8. The selected tone output is supplied to a limiter 48 by means of lines 4I and from the limiter 48 to a filter 59 and is then fed to a tone line connected to recording apparatus at a local or remote point. The filter 50 removes unwanted side frequencies produced in the limiter 48 or by diversity switching in unit 4I). The recording apparatus may be atelegraph printer or a` tone'keyer or the like.

When the carrier is phase modulated by the various tone frequencies and both side bands are supplied to the demodulators I9 and I9', the phase of 'the'carrier supplied to the demodulating converter I9 and I9 must be shifted with respect to the side bands for proper operation, as is knownin the-art.- Then a phase shifting network PS'is included in the connection between the lter 24'and lines 2B and 28 by moving switch S. to the right vhand position.

Theexalted carrier is also used for automatic frequency control purposes and some carrier output from the filter 24 is -supplied to a discriminator` and detector circuit in unit 2'I wherein a potential is developed; which varies with respect to a base potential as the-carrier frequency shifts with respect to its correct value. This potential is supplied bylead 29-to an automatic frequency control tube or mechanical? means in rectangle 3I to control the frequency ofthe intermediate frequency VoscillatorLO". The discriminator and detector in unit 2'Imay be of 'the Seeley or Conrady type or modicationsthereof and will not be described in detail herein, since it is well known in the art. The control mechanism in unit 3I may'include-areactance tube controlled by the potential'developed in 2I"orfa motor the direction of rotation of'which is.: controlled by said potential. Thenth'e'reactance tube may be included in the frequency-determining circuits of theloscillator LO'.' Ony the other hand, a motor may be included in unit 3`I, in which case its direction of rotation isdeterniined by the polarity of the potential out of-discriminator 2'I with respect to said basepotential 'and the motor derives a reactance in the oscillation generator circuit. The discriminator and detector. 2''and control apparatus 3I operate on the generator LO to stabilize the frequency of theroutput of IF and IF' in a wellk'nown '-manner.

Common automatic gain controlcircuits are supplied ,for the receivers and'comprise detectors D and D in the narrow band units I2 and I2 which are excitedby carrier energy to provide across the commonloadresistor LR a potential which dependsonthe carrier strength in the two receivers.. This potential is supplied by resistor RI `to each of-the receivers Rv andv R to control in aawell known'man-ner the. bias on the control electrode of Vone or more amplifier stages therein to regulate or'control the strength of the signal supplied at the mst-intermediate frequency to units IF and IF. Capacitor C and resistor RI are so chosenthat the 'gai-n control action is comparatively'fast and-'the gain of 'each receiver is controlled in such'afmannerto supply signals of good strengthtotheintermediate frequency units IF and IF'.

The apparatus in unit' I 8, as stated above, comprises-means for selecting-thestrongest carrier, means for-producinga'- potential, the polarity of which with respect to acbase potential indicates which carrier is strongest, a lockingpcircuit and means opera-ted- 0r controllediby said potential for trippingsaidlockingr'circuit; The apparatus described in4 thepreceding sentence is included in' rectangle 59 of Figi' 2f is substantially as'disclosed and claimed in saidiSch'ockA-et al; applica- 5,. tion and the description thereof will not be repeated herein. The carriers from the two leads i4 and i4 are supplied to two branches. The branches 60 and 60 supply this carrier to the said carrier strength sensing and selecting means 59. The branches 62 and 62 are coupled by transformers 3G and 66 to amplifying stages or gating stages lil and lil controlled by the output of the locking circuit. In the apparatus included in rectangle 5t, the strongest carrier is selected and the locking circuit is controlled to provide potentials at points a and b which become plus and minus or minus and plus depending upon which carrier is strongest. The carriers supplied by leads 62 and 62 are fed by transformers 65 and 66 to the control grids of push pull gating stages l and lll. When these tubes are con ductive, the carriers are supplied by transformers l2 and 12 to output leads 20. The gating stages l0 and 'I0' are alternatively conductive so that one carrier only supplies input by the lines 20 to limiter 22. The gate control potentials at points a and b are fed differentially to the control grids le and lll of control stages 'I6 and 16 each of which includes a pair of electron discharge devices in cathode follower circuits with cathode load resistcrs 78 and 'I8'. The cathode end of the resistor 'i8 is coupled to the cathodes of the tubes in gating stage l'. The cathode end of the resistor 'i8' is coupled to the cathodes of the tubes in the gating stage l0. A point on the secondary winding of transformers B and 66 is grounded thereby completing the grid biasing circuits for the tubes of stages and 1D. Assume that the carrier on leads i4 is of the best strength and that this carrier is also fed to the leads 32 and thence by stage l0', if conductive, to the leads 20 for use. Then the comparer and selector circuit in unit 59 provides at point b a negative potential which biases the grids 14 to cut off so that the tubes of stage 'I6' do not draw current. Then the potential at the cathode end of resistor 18 is nearer ground potential than it would be if current were flowing in these tubes and as a consequence the cathodes of the tubes of stage la are nearer ground potential while the control grids of these tubes are at substantially ground potential and the tubes of stage lil are conductive provided the resistor 18' is'of the proper value. The tubes of stage 'i0' being conductive, pass the strongest carrier to the line 2U and limiter 22 for use. At the same time, the potential at the point a is positive or at least less negative so that the grids 'I4 are biased in the positive direction and current flows in the tubes of the stage 1B to provide considerable potential drop in the resistor 'i8 and apply a more positive potential to the cathodes of the tubes of stage 76 and the cathodes of the tubes in the gating stage 1G. This is the same as making the control grids of the tubes of the gating stage l0 more negative and this stage is biased to cut off so that the poorer carrier is blocked and is not used in the carrier exalted demodulators I9 and i9.

The apparatus in unit fil) may be similar to this apparatus illustrated in Fig. 2 with changes in circuit elements and constants sufficient to make the circuits appropriate for the frequencies used at the output of the audio channel filters 36, 33 and so forth. There is then no need of illustrating the apparatus in unit 40, since the nature thereof will be clear to those skilled in the art.

The balanced detectors in units I9 and I9 may be as illustrated in Fig. 3 of the drawings. The detector in unit I9 will be described. The side bands and weak carrier are supplied from unit l0 by leads l5 to the primary winding of a trans? former T, the secondary winding of which couples the control grids of a pair of electron discharge devices 8B and 80 in push pull relation. The exalted carrier from the narrow filter 24 is supplied by leads 26 and 28 to the primary Winding of a second transformer Tl, the secondary winding of which is connected to a point on the secondary winding of transformer T to impress'the selected carrier (the best carrier) co-phasally on the control grids of tubes and 80. The secondary winding of transformer T! and its connection also serves as a grid bias circuit for the control grids of tubes 30 and 8D and includes the necessary bias source in the connection between the grids and ground. The cathodes of the tubes 8E! and 83 are grounded to complete this circuit. The carrier is suppressed in the differential output circuit including transformer T2 and the modulation components appear in the secondary winding thereof and are fed by leads 2| to the channel filters, two of which are shown in each detector output and designated 35, 36', 38, 38 and so forth.

The embodiment of Fig. 4 is similaito the embodiment of Fig. l except for these differences. It may be desirable in some cases to operate the selecting apparatus in unit 4i! at higher than audible frequency, in order that the time constants of the circuits involved need not be slowed up too much by virtue of the large capacitor and reactor elements necessary at the low frequencies. Then a frequency multiplier FM is included in the leads between the units 36 and 4i! and a frequency multiplier FM' in the leads between 36 and 40 so that the frequency shifted audio tones may be multiplied up to the desired frequency before selection takes place. Then a selected tone is divided down by frequency dividing circuits FD inserted between the limiter 48 and the lter 5B. The multiplying and dividing circuits may be conventional and details` of the same have not been shown herein.

There is a selector 4B for each channel in Fig.4 l and in Fig. 4. These selectors then select thebest tone on each channel and the selected toneafter limiting at (i8, 48', 48", etc., is filtered at. 50, 53', 5U", etc. and fed over tone lines to thepoint at which it is used. In the embodiment of i Fig. 4, the tones are raised in frequency before selection and reduced in frequency after selec-1 tion and limiting.

What is claimed is:

l. In receiving apparatus for selecting the best of several received versions ofthe same signal, each of which versions comprises a carrier which has been modulated at the transmitter by signals, means for extracting from each version substantially only the carrier, means for ccmparing the strengths of the several extracted carriers and for selecting for use the strongest of the several extracted carriers, and a detector for each version excited thereby and by the selected carrier.

2. Apparatus as recited in claim 1 including means coupled to the detectors to select signal output from that detector supplying the strongest signal output.

3. Apparatus for demodulating a plurality of versions of a signal, each of which versions comprises carrier current and side band energy comprising in combination, a detector for each version of said signal, means for impressing the r carrier current and side band energy of each ver- 7. sion of said-signalen its detector, means. for se-l lecting the strongest unmodulatedcarrier of said several versions of said signal, and means, for impressing the Said Selected carrier on all -o said detectors.

a.A A system. as recited in. claim 3. including other apparatus for comparing the. strength of thel detQCted signals and selecting theA strongest signal.

5 In an. exalted carrier diversity system. a plurality 0f reeeirers eachV comprisingmeans.- tor generating currents representing side band and carrier energy resulting from modulation of. a Carr' Ar by signals, means in each receiver coupled with said last. named means for Separating currents representing the; unmoduiated carrier energyY from each or said representative currents, carrier energy strength comparing; and se lecting means excited by the separate unmodu lated carriers for selecting the strongest carrier, a detector coupled to each receiver, and means for feeding the selected carrier to all or said detectors.

,6, In an exalted carrier diversity system, a plurality of receivers each comprising means for generating currents representing side band and carrier energy resulting from modulation o f carrier by a plurality o f signal modulated tones, means in each receiver coupled with said last named means for separatingI currents repre- I.

senting the unmedulated carrier energy from eachof said representative currents, carrier energy streng-th comparing and selecting means excited by thei separate unmodulated carriersfor selecting the strongestcarrier, a converter cou,.- pled to each receiver, means for feeding the selected carrier to allot said converters,l and a plurality of lters one foreach tone coupled to said converter.

'7. I-n an exalted carrier diversity systemy aplureality of receivers each comprising meansV for generating currents representing side band and carrier energy resulting from` modulation of a main carrier by a plurality ,of signal modulated sub-carriers, means in each receiver coupled, with said last named means for separating currents representing the unmo-dulated carrier energy from each of said` representative currents, carrier current strength comparing and selecting means excited by the several' separate uninodulated carriers for selecting the strongest carrier, a converter in each receiver excitedby the currents generated therein, meansior feeding the selected carrier to all of said converters, a plurality oi filters connected `toeech converter there being corresponding filters one for each sub-carrier coupled to each converter, and means for selecting sub-carrier energy from that filter of the corresponding lters wherein the suo-.carrier is of best strength.

8. In an exalted carrier diversity system, a plurality of receivers each comprising means for generating currents representing side bandhand carrier energy resulting from modulation of a carrier by signals, means in each of. said-receivers for separating currents representing theunmodulated carrier energy from eachA of saidrepresentative currents, a detecting system excited by the separate unmcdulated carrier currents for comparing the strength thereof and' producing a potential the magnitude of which `depends on which of the unmodula-ted-J currents representing the-carriers isstrongest, aY gating system including a valve in each receiver excited by the corresppnding unmod-ulated carrier in each receiver, means for controlling the conductivity of said valves byv said produced potential to pass the strongest 4carrierv only, a detector for each receiver, and connections between said valves and al-l;of the detectors for feeding the selected unmodulated carrier to all of said detectors.

9;. In an exalted` carrier diversity system, a plurality of receivers each comprising resonating; circuits for generating currents representing side band, and carrier energy resulting from modulation oiga main carrier by a plurality of sub-carriers each in turn modulated by signals, a, carrier filter in each receiver for separating frein said currents, currents representing the n rair-i unmodulated carrier, carrier strength comparing detectors coupled to each of said, lters Vforselecting the strongest unmodulated carrier, a converter in each receiver, connections between said carrier strength comparing detectors and the converter in each receiver for supplying thereto the strongest unmodulated carrier, and a plurality o f, filter circuits coupled lto each` said last named converters, there being a iilter circuit for each of the sub-carriers used to. modulate the main carrier.

10,.A In an exalted carrier diversity system, a plurality ci. receivers each comprising means for generating currents representing side band and carrier energy resulting from modulation ol a carrier by a plurality of signal' modulated tones, means in eachof said'receivers for separating Currentsl representing the unmodulated carrier energy from; each of said representative currents, a detecting system excited by the separate unmcdula-ted carrier currents for comparing the strength thereof and producing a potential the magnitude of which depends on which of the currents representing the carriers is strongest, a gating system including a valve in each receiver excited by thecorresponding unmodulated carrier in eachl receiver, means for controlling the conductivity` of said valves by. saidi produced po,u tential to pass the strongest carrier only, a detector foreach receiver excited by the representatiVecurrent genera-ted. therein, connections be.- twieen said valves and' all of the detectors for feeding the selected unmodulated` carrier to all of said detectors, a ulterior each modulated tone coupled to each detector, and means foriselecting outputfrom that oneof the filters which has the strongest tone.

HAROLD O. PETERSON.

REFERENCES CITED The-,following references are of record in the file ofl this patent:

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